Steel strip having a low si content

ABSTRACT

A dual phase or complex phase steel strip showing no tiger stripes. The steel strip having an ultimate tensile strength Rm classifying for 1000 MPa steel category, includes (in mass percent) C 0.09-0.19%; Mn 1.9-2.6%; Si at most 0.1%; Cr 0.4-0.8%; Mo at most 0.3%; Ni at most 0.4%; Al 0.02-1.3%; and optionally one or more of the following elements: Nb at most 0.08%; P equal to or more than 0.0005%; N equal to or less than 0.015%; Ti equal to or less than 0.1%; V equal to or less than 0.1%; B equal to or less than 0.01%; wherein the sum of Cr, Mo and Ni is at least 0.5%; the balance being Fe and inevitable impurities.

The invention relates to a steel strip having a low silicon content, in particular a hot dip galvanised steel strip, more particularly a hot dip galvanised dual phase or complex phase steel strip.

Dual phase (DP) steel strip having balanced properties regarding strength and ductility (formability) is well known in the art. In cold rolled DP steel strip silicon is a very common alloying element for reason of cost reduction and/or control, solid solution strengthening, avoiding carbide precipitation, which is favourable in view of ductility (elongation). However, Si induces surface-related problems. Si oxides formed during processing in the hot strip mill and annealing line increase the risk of the occurrence of so-called “tiger stripes”, which affect appearance and coatability. Therefore, commonly the use of silicon in DP steel is restricted, especially in hot dip galvanized (HDG) DP steel. Typically Si content in DP steel is 0.2-0.3 wt. % in HDG DP steel sheet and 0.2-0.5 wt. % in uncoated steel sheet.

From EP 2169091 A1 a high-strength hot-dip galvanized steel sheet with low yield strength is known. The steel composition comprises, in mass percent, 0.01-0.12% C, 0.2% or less Si, less than 2% Mn, 0.04% or less P, 0.02% or less S, 0.3% or less sol. Al, 0.01% or less N, 0.3-2% Cr, also satisfying the condition 2.1≦[Mneq]≦and 0.24≦[% Cr]/[% Mn], the balance being iron and inevitable impurities. The steel microstructure is composed of ferrite and a second phase, wherein the area ratio of the second phase is 2-25%, the area ratio of pearlite or bainite in the second phase is 0-20%, the average grain diameter of the second phase is 0.9-7 micrometer, and the area ratio of grains with a grain diameter of less than 0.8 micrometer in the second phase is less than 15%. The steel composition is exemplified by a number of examples based on a low Si and low Al alloying philosophy.

It is an object of the present invention to provide a steel composition, such as a dual phase (DP) or complex phase (CP) steel composition, which combines desired microstructure properties regarding strength, in particular an ultimate tensile strength of at least 1000 MPa, and ductility and also with sufficient surface quality, in particular avoiding the occurrence of tiger stripes.

It is another object of the invention to provide such a steel composition having a good coatability.

According to the invention a steel strip having an ultimate tensile strength Rm classifying for 1000 MPa steel category, comprising (in mass percent)

-   -   C 0.09-0.19%;     -   Mn 1.9-2.6%;     -   Si at most 0.1%;     -   Cr 0.4-0.8%;     -   Mo at most 0.3%;     -   Ni at most 0.4%;     -   Al 0.02-1.3%;     -   and optionally one or more elements selected from the group         consisting of:     -   Nb at most 0.08%;     -   P equal to or more than 0.0005%;     -   N equal to or less than 0.015%;     -   Ti equal to or less than 0.1%;     -   V equal to or less than 0.1%;     -   B equal to or less than 0.01%.     -   wherein the sum of Cr, Mo and Ni is at least 0.5%;     -   the balance being Fe and inevitable impurities and inevitable         impurities.     -   Surprisingly it has been found that the steel composition         according to the invention does not show tiger stripes at a high         level of strength and formability properties.

Regarding the metallurgical aspects the following guidelines are deemed to apply:

C is 0.09-0.19%, preferably 0.12-0.17%. If C is above the upper limit, then it becomes difficult to assure good spot weldability. If C is below 0.09%, then the carbon content is too low to create DP or CP microstructures.

Si is at most 0.1%. Si has shown to be the major factor responsible for tiger stripes. In order to avoid the risk of the occurrence of these surface defects in a secure way, the amount is limited to a maximum of 0.1%. Preferably the Si content is equal to or less than 0.08%, more preferably 0.02-0.08%.

Usually the addition of Al and the addition of Si have a similar beneficial effect on the formability of steels. However, in view of tiger stripes Al has a negligible effect. Therefore Al is in the range of 0.02-1.3%. Preferably, Al is in the range of 0.02-0.35%.

Mn is present at a level of 1.9-2.6% thereby compensating the reduction of strength caused by the low Si content, preferably in the range of 2.10-2.50%.

Cr, Mo and Ni are also present for strength and hardening reasons determining the DP or CP microstructure of the steel strip of the invention. According to the invention the total sum of these elements is at least 0.5%. The amounts of the individual elements are

-   -   Cr 0.4-0.8%, more preferably more than 0.50 up to 0.70%;     -   Mo 0-0.3%, preferably less than 0.01%; and     -   Ni 0-0.4%, preferably less than 0.05%.

The steel strip composition according to the invention may optionally comprise is other alloying elements in small amounts. The sum of inevitable impurities and other alloying elements is advantageously less than 1.25%. Preferably the limits of the additional alloying elements and individual impurities are

-   -   P equal to or more than 0.0005% and advantageously in the range         of 0.005-0.05%     -   N at most 0.015%     -   Nb at most 0.08%, more preferably at most 0.04%     -   Ti at most 0.1%     -   V at most 0.1%     -   B at most 0.01%.

In an advantageous embodiment the steel strip according to the invention has a microstructure (area %), comprising

-   -   martensite 5-40%;     -   ferrite/upper bainite 30-95%;     -   lower bainite/tempered martensite 0-30%;     -   and optionally pearlite less than 5%.

Ferrite and martensite are required in view of work hardening and ductility. In CP products the martensite content should not be to high, since it affects yield strength. Bainite provide strength and balances local hardness differences in the microstructure. Advantageously the amount of pearlite is small, preferably about zero. Advantageously the grain size is less than 10 micrometer in view of strength.

In a preferred embodiment the steel strip is coated with a coating comprising Zn. Although the coating can be applied in various ways, hot dip galvanising is preferred using a standard GI coating bath. Other Zn coatings may also be applied. An example comprises a Zn alloy coating according to WO 2008/102009, in particular a zinc alloy coating layer consisting of 0.3-4.0% Mg and 0.05%-6.0% Al and optionally at most 0.2% of one or more additional elements along with unavoidable impurities and the remainder being zinc. An additional element typically added in a small amount of less than 0.2 weight %, could be selected from the group comprising Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. Pb, Sn, Bi and Sb are usually added to form spangles. Preferably, the total amount of additional elements in the zinc alloy is at most 0.2%. These small amounts of an additional element do not alter the properties of the coating nor the bath to any significant extent for the usual applications. Preferably, when one or more additional elements are present in the zinc alloy coating, each is present in an amount <0.02 weight %, preferably each is present in an amount <0.01 weight %. Additional elements are usually only added to prevent dross forming in the bath with molten zinc alloy for the hot dip galvanising, or to form spangles in the coating layer.

The invention will be further illustrated by means of the following examples:

Sheets having a composition as indicated in Table 1 below were manufactured by a usual cold rolling and continuous annealing process. E.g. steel blocks were casted from the composition and subsequently an industrial hot rolling simulation was performed. More specifically, a strip was hot rolled to a desired thickness, such as 4 mm, at a finishing temperature of 880° C. and a coiling temperature of 650° C. After hot rolling the strip was cold rolled to a final thickness of 1 mm and annealed as indicated below.

Example 1 was processed using different annealing treatments leading to 3 distinct tensile property sets as indicated in Table 2. Each treatment (see Table 3) comprises heating of the respective sheet to a maximum temperature T1, following which the sheet was cooled slowly (rate 1-10° C./s) down to the temperature T2. From T2 a rapid cooling (rate 20-60° C./s) occurred to T3, at which temperature the sheet was maintained for some time 40-100 sec.). Then the sheet thus treated was allowed to cool to ambient temperature in air.

For Examples 2-4 cold rolled strip is made via standard steel making and rolling routes as outlined above. After cold rolling the material is continuously annealed in standard annealing lines as outlined above. After heating to a temperature T1, slow cooling to T2 is carried out and subsequently a rapid cooling to temperature T3 (the relevant data are presented in Table 3). Next, the material is either a) further cooled to room temperature in Examples 2a and 3a, or b) for the GI (galvanized) Examples 2b, 2c, 3b and 4a-4c the strip is brought to the Zn-bath temperature, or c) for the GA Example 2d an additional standard GA (galvannealed) heating step is applied.

Table 4 presents the data concerning the microstructure and grain size of Examples 2-4. The CP variants, Examples 2a and 4a, have a microstructure, comprising more than 1% of martensite, more than 10% of ferrite, more than 50% bainite+tempered martensite and less than 5% of pearlite. The microstructure of the DP variants, Examples 2b, 2c, 3a and 3b, was comprised of at least 5% of martensite, more than 50% of ferrite, optionally bainite+tempered martensite and less than 5% of pearlite.

The sheets were also visually examined for the presence of tiger strips. Tiger stripes were not observed. Also a good coatability was achieved.

TABLE 1 Composition Examples 1-4 Σ Example C Mn P S Si N Al Nb B Ti V Cr Mo Ni Cr + Mo + Ni 1 0.14 2.29 0.001 0.0012 0.06 <0.001 0.292 0.0015 0.029 0.54 0.54 2 0.148 2.225 0.011 0.002 0.051 0.0051 0.036 0.015 0.0001 0.001 0.005 0.545 0.003 0.022 0.570 3 0.160 2.178 0.010 0.003 0.057 0.0040 0.031 0.016 0.0001 0.027 0.006 0.540 0.003 0.028 0.571 4 0.129 2.210 0.012 0.002 0.054 0.0031 0.254 0.001 0.0019 0.042 0.007 0.549 0.006 0.031 0.586

TABLE 2 Physical properties Examples 1-4 Thickness Rp Rm A80 Example (mm) (MPa) (MPa) (%) 1a 1.0 748 1094 6.1 1b 1.0 691 1114 7.2 1c 1.0 611 1093 9.5 2a 1.5 864 1042 8.4 2b 1.5 652 1040 11.4 2c 1.5 709 1020 10.5 2d 1.5 609 1003 13.3 3a 1.5 582 1022 10.9 3b 1.5 640 1007 12.7 4a 1.5 896 1013 9.4 4b 1.1 916 1031 7.1 4c 1.5 784 983 11.1

TABLE 3 Annealing data Examples 1-4 Example T1 (° C.) T2 (° C.) T3 (° C.) 1a 850 680 380 1b 840 680 475 1c 840 600 475 2a 860 675 320 2b 800 620 480 2c 820 690 490 2d 850 700 470 3a 810 620 275 3b 800 700 480 4a 850 630 400 4b 860 650 420 4c 840 640 440

TABLE 4 Microstructure Examples 2-4 Example Microstructure Grain size 2a CP <10 2b DP <5 2c DP <5 2d DP 3a DP <8 3b DP <5 4a CP <10 4b CP <10 4c DP 

1. A steel strip having an ultimate tensile strength Rm of at least 1000 MPa, consisting of (in mass percent) C 0.09-0.19%; Mn 1.9-2.6%; Si at most 0.1%; Cr 0.4-0.8%; Mo 0.003-0.3%; Ni at most 0.4%; Al 0.02-1.3%; Nb at most 0.08%; P 0 or equal to or more than 0.0005%; N equal to or less than 0.015%; Ti equal to or less than 0.1%; V equal to or less than 0.1%; B equal to or less than 0.01%; wherein the sum of Cr, Mo and Ni is at least 0.5%; the balance being Fe and inevitable impurities.
 2. The steel strip according to claim 1, wherein Si is at most 0.08%.
 3. The steel strip according to claim 1, wherein (in mass percent) C is 0.12-0.17%; Mn is 2.10-2.50%; Al is 0.02-0.35%.
 4. The steel strip according to claim 1, wherein Cr is 0.50-0.70%; Mo is 0.003 to 0.01%; Ni is at most 0.05%, B is 0.0001 to 0.01%.
 5. The steel strip according to claim 1, comprising wherein Nb is 0.001 to 0.04% P is 0.005 to 0.05%.
 6. The steel strip according to claim 1, having a microstructure consisting of, in area %, martensite 5-40%; ferrite/upper bainite 30-95%; lower bainite/tempered martensite 0-30%; and pearlite less than 5%.
 7. The steel strip according to claim 1, having grain size less than 10 micrometer.
 8. The steel strip according to claim 1, coated with a coating comprising Zn.
 9. The steel strip according to claim 8, being hot dip galvanised.
 10. The steel strip according to claim 8, wherein the coating consists of, in mass percent: 0.3-4.0% Mg and 0.05-6.0% Al and at most 0.2% of one or more additional elements selected from the group of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, along with unavoidable impurities and the remainder being zinc.
 11. The steel strip according to claim 2, wherein (in mass percent) C is 0.12-0.17%; Mn is 2.10-2.50%; Al is 0.02-0.35%.
 12. The steel strip of claim 1, consisting of (in mass percent) C 0.12-0.17%; Mn 2.10-2.50%; Si 0.02-0.1%; Cr 0.50-0.70%; Mo 0.003 to 0.3%; Ni at most 0.4%; Al 0.02-0.35%; V 0.005 to 0.1%; B 0.0001 to 0.01, the balance being Fe and inevitable impurities.
 13. The steel strip of claim 1, wherein Nb is at most 0.04%; P is 0.0005% to 0.05%; N is equal to or less than 0.015%; Ti is equal to or less than 0.1%; V is 0.005 to 0.1%; and B is 0.0015 to 0.01%.
 14. A steel strip having an ultimate tensile strength Rm of at least 1000 MPa, consisting of (in mass percent) C 0.12-0.17%; Mn 2.10-2.50%; Si 0.02-0.1%; Cr 0.50-0.70%; Mo 0.003 to 0.01%; Ni at most 0.05%, wherein Ni is present; Al 0.02-0.353%; Nb at most 0.04%; P 0.005 to 0.05%; N equal to or less than 0.015%; Ti equal to or less than 0.1%; and one or more elements selected from the group consisting of: V 0.005 to 0.1%; B 0.0001 to 0.01%; wherein the sum of Cr, Mo and Ni is at least 0.525%; the balance being Fe and inevitable impurities, having a microstructure consisting of, in area %, martensite 5-40%; ferrite/upper bainite 30-95%; lower bainite/tempered martensite 0-30%; and pearlite less than 5%.
 15. The steel strip of claim 14, wherein B is 0.0015 to 0.01%. 